Combustion exhaust gas treatment process and treatment apparatus

ABSTRACT

Denitrated combustion exhaust gas is desulfurized with ammonia, the waste water obtained from washing the desulfurization waste water or dust ash is filtered using calcium carbonate, the pH of the filtrate is adjusted, carbon dioxide and/or aqueous carbonic acid is added and then a flocculating agent is added, and ammonia is recovered from the supernatant after precipitation and separation of the solid portion. The recovered ammonia is added to the denitrated gas for treatment of the combustion exhaust gas.

TECHNICAL FIELD

[0001] The present invention relates to a combustion exhaust gastreatment process and apparatus.

BACKGROUND ART

[0002] Exhaust gas treatment apparatuses for thermal power boilers arecommonly equipped with denitration apparatuses designed to removenitrogen oxides (NO_(x)), electric dust collectors designed to removedust, and desulfurization apparatuses designed to remove sulfur oxides(mainly SO₂).

[0003] In cases where the thermal power boiler employshigh-sulfur-containing heavy oils and petroleum coke as fuels, it isknown that the exhaust gas contains SO₂ as well as SO₃ and/or sulfuricacid mist. The SO₃ and/or sulfuric acid mist contained in the exhaustgas cannot be easily removed with a desulfurization apparatus usingcalcium carbonate, and their removal has therefore been accomplished byinjecting ammonia into the downstream exhaust gas of the denitrationapparatus to convert it into harmless ammonium sulfate or ammoniumhydrogen sulfate (hereunder also referred to as “acid ammoniumsulfate”), which is then separated and removed with an electric dustcollector and desulfurization apparatus.

[0004] The ammonia supplied to the downstream end of the denitrationapparatus must be supplied in excess of the stoichiometric amount forcomplete removal of the SO₃ and/or sulfuric acid mist, and the unreactedammonia supplied in excess is absorbed in the desulfurization apparatusat a later stage. Most of the ammonium sulfate or acid ammonium sulfate,as a reaction product of the SO₃ and/or sulfuric acid mist and ammonia,is removed by the electric dust collector at a later stage, and theunremoved ammonium sulfate or acid ammonium sulfate is absorbed at thedesulfurization apparatus together with the unreacted ammonia. Thus, thewaste water discharged from the desulfurization apparatus contains anabundant nitrogen portion in the form of ammonia, ammonium sulfate andacid ammonium sulfate, and without treatment, such waste water cannot bedisposed of under the recently enforced waste water regulations.

[0005] The dust ash that has been separated and removed by the electricdust collector also contains ammonium sulfate and acid ammonium sulfate,and since the waste water used to wash this dust ash contains anabundant nitrogen portion in the form of ammonia, ammonium sulfate andacid ammonium sulfate, this waste water also cannot be disposed of underthe current nitrogen waste water regulations, unless it is treated. Ithas therefore been the practice to reduce the nitrogen content of wastewater to within regulation standards using a method in which thenitrogen portion is removed through denitration by biological treatmentusing nitrifying bacteria in the waste water treatment apparatus.However, the method of using nitrifying bacteria for biologicaltreatment of the nitrogen portion dissolved in waste water poses theproblem of a large treatment apparatus volume and increased equipmentcosts, making it necessary to reduce the amount of ammonia added forremoval of the SO₃ and/or sulfuric acid mist.

[0006] In order to reduce the amount of ammonia used, it is desirable torecover and recycle ammonia from the ammonia-containing waste water ofthe ammonia-containing desulfurization apparatus and theammonia-containing waste water used to wash the dust ash. However, afterseparation of the calcium sulfate produced upon desulfurization by thelimestone-gypsum method employing calcium carbonate, the filtrate has ahigh Ca content, and when this waste water is introduced into an airheater or stripper for the ammonia recovery procedure, Ca precipitatesin and blocks the evaporator or stripper, causing a problem which isprohibitive to prolonged continuous operation.

[0007] It has therefore become a common procedure that after the gypsum(CaSO₄) produced in the desulfurization waste water is separated by afilter, the pH is adjusted with an alkali metal hydroxide such as NaOH,and the Ca portion in the filtrate is precipitated and removed togetherwith a coagulant. Still, the filtrate to which the coagulant has beenadded after pH adjustment for precipitation and separation usuallycontains at least 200 ppm of Ca, most of which is in the form ofCa(OH)₂. When this solution is supplied to an evaporator or stripper forheating, the trace amount of ammonium carbonate included thereinreleases carbonic acid, and the released carbonic acid reacts with theCa(OH)₂ to precipitate calcium carbonate, thus creating a problem whichprohibits prolonged continuous operation.

DISCLOSURE OF THE INVENTION

[0008] It is an object of the present invention, which has beenaccomplished under these circumstances, to provide a combustion exhaustgas treatment process and apparatus, employing a process for treatmentof combustion exhaust gas in which the ammonia contained in waste waterobtained after desulfurization or waste water used to wash dust ash isrecovered, and the recovered ammonia is added to the denitrated gas.

[0009] As a result of diligent research aimed at achieving this object,the present inventors succeeded in completing the present invention uponfinding that the aforementioned object can be achieved by removingammonia from a supernatant obtained by filtering a solution which hasbeen desulfurized using an aqueous solution containing calciumcarbonate, adjusting the pH of the filtrate to 9-12 while simultaneouslyadding carbon dioxide and/or an aqueous carbonic acid solution, or elseadjusting the pH to 9-12 after adding carbon dioxide and/or an aqueouscarbonic acid solution, and then adding a flocculating agent toprecipitate and separate a solid portion from the supernatant.

[0010] Thus, the present invention provides a combustion exhaust gastreatment process which comprises adding ammonia to denitratedcombustion exhaust gas to convert S0 ₃ and/or sulfuric acid mist toammonium sulfate and/or ammonium hydrogen sulfate, removing the producedammonium sulfate and/or ammonium hydrogen sulfate with dust,desulfurizing the resulting gas using an aqueous solution containingcalcium carbonate and filtering the solution to separate calcium sulfateand filtrate, and then adjusting the pH of the filtrate to 9-12 whilesimultaneously adding carbon dioxide and/or an aqueous carbonic acidsolution, or else adjusting the pH to 9-12 after adding carbon dioxideand/or an aqueous carbonic acid solution, adding a coagulant toprecipitate and separate a solid portion to form a supernatant,conducting the supernatant to a separately provided ammonia recoverystep at which ammonia is recovered by introducing steam for distillationand concentration, and thereafter adding the recovered ammonia to theaforementioned denitrated gas.

[0011] The invention further provides a combustion exhaust gas treatmentapparatus comprising at least an electric dust collector for removal ofdust, a desulfurization apparatus employing calcium carbonate and awaste water treatment apparatus for treatment of the desulfurized wastewater discharged from the desulfurization apparatus, the combustionexhaust gas treatment apparatus being constructed in such a manner thatammonia recovered from the desulfurized waste water using a waste watertreatment apparatus comprising the following apparatuses (1) to (4)returns to the upstream end of the electric dust collector.

[0012] (1) A filtering apparatus for filtration of calcium sulfate.

[0013] (2) A pH adjusting apparatus provided with a function ofsupplying carbon dioxide and/or an aqueous carbonic acid solution.

[0014] (3) A precipitation and separation apparatus which precipitatesand separates the solid portion after addition of the coagulant.

[0015] (4) A concentration apparatus which introduces steam into thesupernatant obtained by precipitation and separation of the solidportion, and recovers the ammonia.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 is a schematic view illustrating an example of thecombustion exhaust gas treatment process of the invention.

[0017]FIG. 2 is a schematic view illustrating another example of thecombustion exhaust gas treatment process of the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

[0018] The combustion exhaust gas treatment process of the invention maybe used for treatment of combustion exhaust gas discharged from boilers,for example, in which case ammonia is first added to the denitratedcombustion exhaust gas to convert the SO₃ and/or sulfuric acid mist toammonium sulfate and/or ammonium hydrogen sulfate, and the producedammonium sulfate and/or ammonium hydrogen sulfate are removed out withthe dust. This is then desulfurized using an aqueous solution containingcalcium carbonate and the solution is then filtered to separate calciumsulfate and filtrate, the pH of the filtrate is adjusted to 9-12 whilesimultaneously adding carbon dioxide and/or an aqueous carbonic acidsolution, or else the pH is adjusted to 9-12 after adding carbon dioxideand/or an aqueous carbonic acid solution, a flocculating agent is addedso that the solid portion is precipitated and separated to form asupernatant. Then, the supernatant is conducted to a separately providedammonia recovery step at which ammonia is recovered by introducing steamfor distillation and concentration and the recovered ammonia is added tothe denitrated gas.

[0019] The method used for denitration treatment of the boilercombustion exhaust gas is not particularly restricted, and for example,it may involve using ammonia in the presence of a catalyst for treatmentof the NO_(x) portion. After denitration, ammonia is added to convertthe SO₃ and/or sulfuric acid mist to ammonium sulfate and/or ammoniumhydrogen sulfate, and the ammonium sulfate and/or ammonium hydrogensulfate produced by reaction between ammonia and SO₃ and/or sulfuricacid mist are collected with the dust. The gas from which most of thesolid portion has been removed is then treated using an aqueous solutioncontaining calcium carbonate for desulfurization.

[0020] After desulfurization, the produced gypsum (CaSO₄) is filteredout and an alkali metal hydroxide is used for adjustment of the pH ofthe filtrate to 9-12 while simultaneously adding carbon dioxide and/oran aqueous carbonic acid solution or else for adjustment of the pH to9-12 after adding carbon dioxide and/or an aqueous carbonic acidsolution, and a coagulant is added to precipitate and separate the Caportion as calcium carbonate. As mentioned above, in conventionalprocesses in which a coagulant is added after pH adjustment, the Caportion still remains in an amount of about at least 200 ppm, but byemploying the process of the invention wherein the pH of the filtrate isadjusted to 9-12 while simultaneously adding carbon dioxide and/or anaqueous carbonic acid solution or else the pH is adjusted to 9-12 afteradding carbon dioxide and/or an aqueous carbonic acid solution, toconvert the Ca portion to calcium carbonate, and a flocculating agent isfurther added for treatment, it is possible to reduce the Ca portion tounder 5 ppm and thus prevent sedimentation of the Ca portion in theammonia recovery step.

[0021] Furthermore, by adjusting the pH to 9-12 it is possible toestablish a condition in which the Ca portion of the waste water isprecipitated and separated as a carbonate while facilitating strippingof ammonia in the recovery step. The yield of ammonia in the recoverystep can be adjusted by the amount of steam supplied, and theconcentration of ammonia remaining in the waste water from the recoverystep can be reduced to under 40 ppm.

[0022] The combustion exhaust gas treatment process of the invention mayalso be one wherein dust collected together with ammonium sulfate and/orammonium hydrogen sulfate produced by addition of ammonia to thedenitrated gas is mixed with water and heavy oil, the mixture isseparated into the solid portion containing the dust and heavy oil andthe aqueous solution containing ammonium sulfate and/or ammoniumhydrogen sulfate, the pH of the aqueous solution is adjusted to 9-12,and then a flocculating agent is added, the solid portion isprecipitated and separated and the supernatant is conducted to anammonia recovery step. When the treatment process of the invention isused for treatment of boiler combustion exhaust gas, it may also employthe solid portion as fuel.

[0023] The combustion exhaust gas treatment apparatus of the inventionis provided with at least an electric dust collector for removal ofdust, a desulfurization apparatus employing calcium carbonate and awaste water treatment apparatus for treatment of the desulfurized wastewater discharged from the desulfurization apparatus, and may be used fortreatment of the SO₃ and/or sulfuric acid mist contained in combustionexhaust gas discharged from a boiler using ammonia, whereby ammonia isrecovered from waste water from the desulfurization apparatus using awaste water treatment apparatus comprising (1) a filtering apparatus forfiltration of calcium sulfate, (2) a pH adjusting apparatus providedwith a function of supplying carbon dioxide and/or an aqueous carbonicacid solution, (3) a precipitation and separation apparatus whichprecipitates and separates the solid portion after addition of thecoagulant and (4) a concentration apparatus which introduces steam intothe supernatant obtained by precipitation and separation of the solidportion and recovers ammonia, and ammonia is returned to the upstreamend of the electric dust collector. In the pH adjusting apparatus of(2), the filtrate obtained after filtration of the calcium sulfate isadjusted to a pH of 9-12, and the method used to accomplish this may bea method of adjusting the pH to 9-12 while simultaneously adding carbondioxide and/or an aqueous carbonic acid solution, or a method ofadjusting the pH to 9-12 after adding carbon dioxide and/or an aqueouscarbonic acid solution.

[0024] The combustion exhaust gas treatment process and treatmentapparatus of the invention will now be explained with reference to theattached drawings. FIG. 1 and FIG. 2 are both outline schematicillustrations showing combustion exhaust gas treatment processesaccording to the invention for treatment of combustion exhaust gasdischarged from a boiler, but the invention is not limited to theseprocesses or apparatuses.

[0025] In FIG. 1, the combustion exhaust gas from a boiler 1 is firstdenitrated at a denitration apparatus 2, after which ammonia 34 andrecovered ammonia 31 are supplied for removal of the SO₃ and/or sulfuricacid mist in the exhaust gas, whereby the SO₃ and/or sulfuric acid mistare converted to ammonium sulfate and/or acid ammonium sulfate. Here,the ammonia is supplied in a proportion of 2-2.5 with respect to the SO₃and/or sulfuric acid mist. The ammonium sulfate and/or acid ammoniumsulfate produced in the exhaust gas are sequestered at the electric dustcollector 3 of the next stage as dust ash 21 along with the unburnt ashin the combustion exhaust gas. The exhaust gas from which the solidportion has been removed at the electric dust collector 3 containsunreacted ammonia that has been supplied in excess of the stoichiometricamount, and this exhaust gas is supplied to a desulfurization apparatus4. At the desulfurization apparatus 4, calcium carbonate 22 iscirculated as an absorbing agent to absorb the unreacted ammonia alongwith SO₂, and the inert portion of the exhaust gas is discharged asatmospheric escape gas 23. The absorption solution is supplied to afiltering apparatus 5 where the gypsum (calcium sulfate) 24 produced isseparated and discharged out of the system, but the filtrate 25 is sentto an ammonia recovery system comprising a pH adjusting tank 6, aflocculation reaction tank 7, a precipitation/separation tank 8 and astripper 9.

[0026] At the pH adjusting tank 6 of the ammonia recovery system, the pHis adjusted to 9-12 with an aqueous sodium hydroxide solution 26 tofacilitate precipitation of the Ca portion of the filtrate 25 in theprecipitation/separation tank 8. This pH condition not only facilitatesprecipitation and separation of the Ca but also facilitates dissipationof the ammonia in the stripper 9. A liquid containing an aqueouscarbonic acid solution 27 is also supplied at the pH adjusting tank 6for precipitation and separation of the Ca portion. The liquid leavingthe pH adjusting tank 6 is supplied to the flocculation reaction tank 7where it is mixed with a coagulant 28. The liquid leaving theflocculation reaction tank 7 is supplied to the precipitation/separationtank 8, where salts including calcium carbonate and heavy metals areprecipitated and separated, and are discharged from the bottom of theprecipitation/separation tank 8 out of the system as solids. Next, thesupernatant 29 is supplied to the stripper 9, and steam 32 is blown infrom the bottom of the stripper 9, ammonia 31 is recovered as an aqueoussolution at the top of the stripper column while ammonia is also removedfrom the bottom of the column and the nitrogen portion is discharged outof the system as waste water 33 meeting waste water regulations.

[0027] As explained above, a stoichiometric excess of ammonia issupplied for removal of the SO₃ and/or sulfuric acid mist contained inthe gas discharged from the denitrating apparatus 2; however, theammonia is not discharged into the atmosphere from the desulfurizationapparatus 4 but is absorbed in the desulfurizing circulation liquid. Thecalcium sulfate produced in the desulfurizing circulation liquid isfiltered out, and then the treatment process of the invention may beused to continuously recover ammonia by reducing the Ca concentration tounder 5 ppm, thus allowing the recovered ammonia to be recycled forremoval of SO₃ and/or sulfuric acid mist, whereby the amount of freshammonia supply may be reduced and the process may thus be rendered moreeconomical. The waste water discharged out of the system from the bottomof the stripper at the same time also sufficiently clears waste waterregulations.

[0028] In FIG. 2, which shows a situation similar to FIG. 1, combustionexhaust gas from the boiler 1 is denitrated at the denitration apparatus2, after which the SO₃ and/or sulfuric acid mist in the exhaust gasusing ammonia 34 and recovered ammonia 31, and the produced ammoniumsulfate and/or acid ammonium sulfate are sequestered at the electricdust collector 3 as dust ash 21 along with the unburnt ash in thecombustion exhaust gas. The dust ash 21 containing ammonium sulfateand/or acid ammonium sulfate is conducted to a mixing tank 10 and mixedwith water 35.

[0029] The mixing tank 10 is provided with a stirrer, and the mixture ofdust ash 21 and water 35 is sent to an ash granulator 11 for mixturewith a binder (heavy oil) 36 and granulation of the ash into a formsuitable for supply to the boiler. The granulated ash 37 is then sent toa water separator 12 for separation of the water, and after separatingthe granulated ash 37 and water 38, the granulated ash 37 is recycled asboiler fuel. The water 38 contains ammonia and ammonium sulfate and/oracid ammonium sulfate, and a coagulant 28 is added upon adjustment to apH of 9-12, after which the solid portion is precipitated and separatedand the supernatant is sent to an ammonia recovery system.

[0030] The present invention will now be explained in further detail byway of the following example which is not, however, intended to restrictthe invention in any way.

EXAMPLE 1

[0031] To denitrated exhaust gas containing SO₃ and/or sulfuric acidmist at about 100 vol ppm (based on dry gas) there was supplied ammoniain an amount corresponding to 2-2.5 times the molar amount of the SO₃and/or sulfuric acid mist. The unreacted ammonia was about 50 vol ppm(based on dry gas). Most of the SO₃ and/or sulfuric acid mist thatreacted with the ammonia was converted to ammonium sulfate or acidammonium sulfate, and these were removed with the dust.

[0032] The exhaust gas containing the unreacted ammonia was conducted toa slurry-like calcium carbonate aqueous solution to convert the SO₂ togypsum, while the unreacted ammonia was absorbed in a circulatingabsorption solution. The SO₃ and/or sulfuric acid mist in theatmospheric exhaust gas discharged after desulfurization was about 2 volppm (based on dry gas).

[0033] A portion of the absorption solution used for desulfurization wasthen extracted and the produced gypsum was filtered out. This extractedsolution contained ammonia at about 9000 ppm, and after removal of thegypsum, the pH of the filtrate was adjusted to 10-12 with an aqueoussodium hydroxide solution. The pH-adjusted solution contained Ca atabout 300 ppm, and since this can cause blockage when supplied to astripper unless the content is reduced, an aqueous carbonic acidsolution was supplied to convert the Ca to calcium carbonate. Theaqueous carbonic acid solution was supplied here in an amountcorresponding to 10 times the amount of Ca. When this solution was mixedwith a flocculating agent to precipitate the solid portion, the Cacontained in the supernatant was reduced to under 5 ppm. After heatingthe supernatant in a heat exchanger, it was supplied to the ammoniarecovery step.

[0034] The conditions for ammonia recovery were in the range of ordinarypressure to 0.1 kg/cmG, and an aqueous ammonia solution with an ammoniacontent of 5-8 wt % was recovered. After gasifying the recoveredammonia, it was supplied to the denitrated gas for removal of the SO₃and/or sulfuric acid mist.

[0035] As a result, it was possible to reduce the amount of ammoniasupplied from outside, while the ammonia in the discharged atmosphericexhaust gas was under 1 vol ppm and the ammonia nitrogen in the wastewater discharged to the outside was under 40 ppm.

[0036] Industrial Applicability

[0037] As explained above, by adjusting the pH of denitrated waste waterand supplying carbon dioxide and/or an aqueous carbonic acid solution inan exhaust gas treatment procedure in which ammonia is supplied toremove SO₃ and/or sulfuric acid mist contained in combustion exhaustgas, it is possible to remove the dissolved Ca portion as calciumcarbonate and thus allow continuous operation of the stripper withoutcausing blockage due to sedimentation of Ca in the stripper. Inaddition, the ammonia can be recovered from the column top and recycledfor removal of the SO₃ and/or sulfuric acid mist in order to reduce theamount of ammonia necessary for use and produce treatment waste waterfrom the column bottom which clears nitrogen content regulations.

1. A combustion exhaust gas treatment process which comprises addingammonia to denitrated combustion exhaust gas to convert SO₃ and/orsulfuric acid mist to ammonium sulfate and/or ammonium hydrogen sulfate,removing the produced ammonium sulfate and/or ammonium hydrogen sulfatewith dust, desulfurizing the resulting gas using an aqueous solutioncontaining calcium carbonate and filtering the solution to separatecalcium sulfate and filtrate, and then adjusting the pH of the filtrateto 9-12 while simultaneously adding carbon dioxide and/or an aqueouscarbonic acid solution, or else adjusting the pH to 9-12 after addingcarbon dioxide and/or an aqueous carbonic acid solution, adding aflocculating agent to precipitate and separate a solid portion to form asupernatant, conducting the supernatant to a separately provided ammoniarecovery step at which ammonia is recovered by introducing steam fordistillation and concentration, and thereafter adding the recoveredammonia to the aforementioned denitrated gas.
 2. The process of claim 1,wherein the combustion exhaust gas is combustion exhaust gas from aboiler.
 3. The process of claim 1 or 2, wherein the removed ammoniumsulfate and/or ammonium hydrogen sulfate together with the dust is mixedwith water and heavy oil, the mixture is separated into the solidportion containing the dust and heavy oil and the aqueous solutioncontaining ammonium sulfate and/or ammonium hydrogen sulfate, the pH ofthe aqueous solution is adjusted to 9-12, and then a coagulant is added,the solid portion is precipitated and separated and the supernatant isconducted to the ammonia recovery step.
 4. The process of claim 1 or 2,wherein the Ca concentration of the supernatant conducted to the ammoniarecovery step is no greater than 5 ppm.
 5. The process of any one ofclaims 1 to 4, wherein the ammonia concentration in the waste waterwhich has been conducted to the ammonia recovery step and has hadammonia recovered is no greater than 40 ppm.
 6. A combustion exhaust gastreatment apparatus comprising at least an electric dust collector forremoval of dust, a desulfurization apparatus employing calcium carbonateand a waste water treatment apparatus for treatment of the desulfurizedwaste water discharged from the desulfurization apparatus, thecombustion exhaust gas treatment apparatus being constructed in such amanner that ammonia recovered from the desulfurized waste water using awaste water treatment apparatus comprising the following apparatuses (1)to (4) returns to the upstream end of the electric dust collector. (1) Afiltering apparatus for filtration of calcium sulfate. (2) A pHadjusting apparatus provided with a function of supplying carbon dioxideand/or an aqueous carbonic acid solution. (3) A precipitation andseparation apparatus which precipitates and separates the solid portionafter addition of the flocculating agent. (4) A concentration apparatuswhich introduces steam into the supernatant obtained by precipitationand separation of the solid portion, and recovers the ammonia.